A glass preform for use in the fabrication of an optical fiber comprises a core and a cladding surrounding the core. The core must have a higher refractive index than the cladding so as to allow easy propagation of light therethrough.
In order to increase the refractive index of the core higher than that of silica, additives such as TiO.sub.2, GeO.sub.2 and Al.sub.2 O.sub.3 are usually added to the core material. In a usual optical fiber, pure quartz glass is often used to form the cladding. In this case, pure quartz glass is produced so that n=1.4585 and .DELTA.n=0.
Referring to FIGS. 1A and 1B, there are shown diagrams illustrating distributions of the refractive index of two types of optical fibers. In these figures, the regions A and B indicate the core and cladding, respectively. The difference in refractive index between the core and cladding is usually indicated in terms of a relative refractive index difference (in percent). That is, assuming that the refractive indices of the core and cladding are n.sub.1 and n.sub.2, respectively, the relative refractive index difference .DELTA.n.sub.12 % is represented by the following equation: ##EQU1##
FIG. 1A shows the general distribution of refractive index of a single mode optical fiber. In this case, .DELTA.n.sub.12 is usually 0.3 to 0.5%. FIG. 1B shows the general distribution of refractive index of a multi-mode optical fiber. For an optical fiber for ordinary communication purposes, .DELTA.n.sub.12 is usually about 1%, and for large aperture optical fibers used in computer link communication applications, .DELTA.n.sub.12 is usually about 2 to 4%.
Oxide additives such as GeO.sub.2 added to increase refractive index of the core cause light scattering (Rayleigh scattering) because of their inherent characteristics. As the amount of the additive added is increased, the degree of light scattering (Rayleigh scattering) due to the additive increases. This is not desirable for light transmission.
If the additive is added in a large amount, bubbles and/or a crystal phase are formed in the glass preform. In the case of GeO.sub.2, for example, GeO gas easily forms, thereby producing bubbles. In the case of Al.sub.2 O.sub.3, clusters of Al.sub.2 O.sub.3 crystals easily forms. This is not desirable for light transmission characteristics and also for the strength of the final optical fiber. Furthermore, the coefficient of thermal expansion of glass increases, which makes the glass preform fragile. Therefore, also from the viewpoints of light propagation and glass strength, it is preferred to reduce the amount of the additive added to the core.
For this reason, it is proposed to increase the refractive index difference between the core and cladding by lowering the refractive index of the cladding. For example, additives which lower the refractive index, such as B.sub.2 O.sub.3, fluorine or a combination thereof, can be added to the cladding. B.sub.2 O.sub.3, however, has disadvantages in that the coefficient of thermal expansion of the resulting cladding greatly changes with the concentration of B.sub.2 O.sub.3 and in that the refractive index changes upon heating. Furthermore, with regards to light transmission characteristics, the cladding has an absorption loss due to B.sub.2 O.sub.3 in a longer wavelength region. Thus, it is preferred to use fluorine as a refractive index-lowering agent.
It is known that addition of fluorine to quartz glass makes it possible to produce optical fibers with various refractive index distributions, and that, by the proper choice of structure, there can be obtained an optical fiber of low dispersion over a wide wavelength region.
The advantage that can be obtained by using fluorine as an additive is that, since the refractive index of the cladding can be made lower than that of pure quartz, pure quartz or quartz glass with a small amount of additive added thereto can be used in the fabrication of the core.
FIGS. 2A through 2D show typical refractive index distribution structures of which those of FIGS. 2A and 2C are of the step index type and those of FIGS. 2B and 2D are of the graded index type. In all of FIGS. 2A to 2D, fluorine is added to the cladding. With regards to the core, in the case of FIG. 2A, a small amount of an oxide which increase the refractive index, such as GeO.sub.2 and P.sub.2 O.sub.5, is added to quartz glass, whereas in the case of FIG. 2C, highly pure quartz glass containing no additive is used. In FIG. 2B, the amount of fluorine added is decreased continuously from the periphery of the core to the center, and the central portion is made of pure quartz glass not containing fluorine (the refractive index of pure quartz glass in n=1.4585, .DELTA.n=0). In FIG. 2D, the amount of fluorine added is decreased continuously from the periphery of the core to the center, and at a certain distance from the periphery, there starts the addition of an additive used to increase the refractive index of quartz glass, with the amount of the additive added increasing continuously toward the center.
As a matter of course, to control the refractive index and facilitate the working of the glass, additives such as GeO.sub.2, P.sub.2 O.sub.5, B.sub.2 O.sub.3 and Al.sub.2 O.sub.3 can be used in combination with fluorine in the cladding and core.
In order to obtain the same refractive index difference as shown in FIG. 1 for an optical fiber of quartz glass with fluoride added thereto, it is sufficient to decrease the amount of oxides added to the core, or alternatively, not to add the oxides at all. This leads to reduction in the degree of Rayleigh scattering due to the presence of the additive. Thus, the resulting optical fiber is preferred as a wave guide. Fluorine is available in abundance as compared with additives such as GeO.sub.2, and furthermore is advantageous from an economical standpoint in that its purification is easy. Another feature is that a fluorine-containing compound is superior not only as a starting material for the additive, but also as a dehydrating agent for removing water contained in the soot.
Various techniques are known for fabrication of quartz glass optical fibers, including the inside chemical vapor deposition (CVD) method (cf. for example, Japanese Patent Publications Nos. 23186/76 and 22423/80), the outside chemical vapor deposition (CVD) method (cf. for example, Japanese Patent Kokai Publication (unexamined) No. 10055/74), the vapor axial deposition (VAD) methods (cf. for example, Japanese Patent Kokai Publication (unexamined) No. 71316/76), and the plasma chemical vapor deposition (CVD) method (cf. for example, Japanese Patent Kokai Publication (unexamined) No. 54446/76). Of these methods, the outside CVD method utilizing flame hydrolysis and the VAD method are superior in productivity and are economical procedures. On the other hand, although fluorine can be added to quartz glass by a procedure utilizing flame hydrolysis, it is quite difficult to uniformly add a sufficient amount of fluorine to the quartz glass by this procedure.
Japanese Patent Publication No. 15682/80 discloses a method in which fluorine is added to glass by supplying a gaseous fluorine-containing compound in the step of synthesizing glass in a gas phase. This method does permit the addition of fluorine to glass, but has a disadvantage in that the efficiency of deposition of glass and the yield of addition of fluorine (doping yield) are low. The reason for this is considered as follows:
In the flame hydrolysis method using an oxyhydrogen flame, water in the flame reacts with a fluorine-containing compound (e.g., SF.sub.6) according to equation (3) below, thereby producing hydrogen fluoride (HF) gas: EQU SF.sub.6 +3H.sub.2 O.fwdarw.SO.sub.3 +6HF (1)
HF gas is stable, and almost all of the fluorine-containing compound is converted into HF gas at elevated temperatures as long as there is water present. Thus, a minor proportion of fluorine-containing compound is utilized as the starting additive material.
HF acts to corrode glass, particularly quartz (SiO.sub.2), and easily reacts with fine quartz particles formed in the flame according to the following equations (4) and (5): EQU SiO.sub.2 (s)+2HF(g).fwdarw.SiOF.sub.2 (g)+H.sub.2 O(g) (2) EQU SiO.sub.2 (s)+4HF(g).fwdarw.SiF.sub.4 (g)+2H.sub.2 O(g) (3)
wherein (s) and (g) indicate solid and gas states, respectively. These reactions inhibit the grain growth of glass particles and decrease the amount of fine glass particles being deposited. This is apparent from the fact that, as the amount of a fluorine compound added is increased, the rate of deposition of fine glass particles drops, and finally they do not deposit at all.
Japanese Patent Kokai Publication (unexamined) No. 67533/80 discloses a method which is intended to overcome the above-described problems of the method of Japanese Patent Publication No. 15682/80. Specifically, it discloses: (1) a method for producing a glass material for optical glass particles formed by the flame hydrolysis method in an atmosphere of a fluorine-containing compound at 1,000.degree. C. or less, and thereafter sintering the laminated body by heating it to higher than 1,400.degree. C. in an inert gas atmosphere; and (2) a method for producing a glass material for optical transmission which comprises heating the glass particle laminated body of (1) in a fluorine-containing compound/inert gas atmosphere to higher than 1,400.degree. C. to form a glass material containing fluorine. Methods (1) and (2) enable the addition of fluorine more effectively than the method of Japanese Patent Publication No. 15682/80. It has been revealed, however, that methods (1) and (2) still have disadvantages as described below.
In method (1), the rate of addition of fluorine to glass is low and, in some cases, the ultimate optical fiber contains impurities such as copper and iron, and an increase in attenuation of light transmission due to such impurities reaches about 3 to 5 dB/km at a wavelength of 1.30 micrometer (the usual loss at this wavelength band is 0.3 dB/km). The amount of fluorine added to glass by method (1) is -0.20% in terms of the refractive index difference .DELTA.n.sub.12 (F).
Method (2) is an efficient procedure in that, as compared with method (1), the rate of addition of fluorine is high and the amount of fluorine added is large. After a processing time of 6 hours, .DELTA.n.sub.12 (F) reaches -0.25%. However, the obtained glass preform is seriously corroded and has an irregular surface. The core tube used in the production of the glass preform, which is a quartz muffle tube used to hold therein a gas atmosphere, is seriously corroded and, in some cases, perforations are formed in the walls of the tube. This etching is considered to partly accelerate the incorporation of impurities from the muffle tube into the soot preform. The attenuation of light transmission of the thus produced optical fiber is about 10 dB/km at a wavelength of 1.30 micrometer. Since the content of hydroxyl groups in the optical fiber is 0.05 ppm or less, it cannot be considered that the increase in absorption loss at 1.30 micrometer is due to the presence of the hydroxyl groups. There are many experiments supporting the conclusion that the increase in absorption loss due to impurities such as copper and iron existing in the optical fiber amounts to 9.5 dB/km.
In addition, the optical fiber produced by the above-described method has disadvantages in that the absorption loss due to the hydroxyl groups changes with time, and that as the temperature rises, the absorption loss considerably increases.
One of the reasons why impurities such as copper and iron are present in the optical fiber is that corrosion of the core tube allows Fe.sub.2 O.sub.3 and CuO present in the core tube walls to migrate to the surface of the tube and to intermingle with the soot, undergoing reactions represented by the following equations: EQU Fe.sub.2 O.sub.3 +2F.sub.2 .fwdarw.2FeF.sub.2 +3/2O.sub.2 ( 4) EQU CuO+1/2F.sub.2 .fwdarw.CuF+1/2O.sub.2 ( 5)
Although FeF.sub.2 and CuF are solid up to 1,100.degree. C., they sublimate at temperatures higher than 1,100.degree. C., thereby intermingling with the soot. Thus the soot preform is contaminated with FeF.sub.2 and CuF.
When Fe.sub.2 O.sub.3 and CuO are contained in the soot preform, even if they undergo the reactions of equations (4) and (5), the resulting products FeF.sub.2 and CuF are not removed from the soot and remain therein as impurities since they are solid at temperatures below 1,100.degree. C. Thus, in accordance with either of methods (1) and (2), impurities are left in the optical fibers.